This invention relates to methods for cleaning semiconductor wafers. More specifically, the invention relates to methods for removing photoresist and post-etch residue from semiconductor wafers. Even more specifically, the invention relates to wafer cleaning methods that utilize supercritical fluid processing.
Wafer cleaning in modern VSLI semiconductor processing presents numerous engineering dilemmas. One important issue involves removal of contamination before, during, and after fabrication steps. For example, photoresist strip and residue removal are critical processes in integrated circuit (IC) fabrication. During dielectric etching in a typical integrated circuit fabrication process, undesirable etch residues and/or polymers such as hydrocarbon, fluorocarbon, and/or polymeric residues (e.g. CxHxFxOx) are formed and left on the surfaces and sidewalls of the resulting structures. Such undesirable residues along with the remaining post-etch photoresist must be removed to prevent quality issues in subsequent deposition process such as, adhesion problems, and/or diffusion contamination
One common method to remove such residues is plasma stripping, however plasma stripping is often damaging to advanced low-k materials. Consequently, non-plasma methods for removing photoresist, residue, and other contaminants from semiconductor substrates are needed.
Conventional non-plasma methods for removal of, for example, post-etch photoresist and polymer residue (especially over low-k dielectrics) present numerous challenges. Traditional wet chemical cleaning methods use solvents such as NMP, along with amines (e.g., hydroxylamine) to strip resist and remove sidewall residue. However such wet solvent processes require a deionized (DI) water rinse to remove traces of solvent from the features that have been etched into the dielectric. As feature sizes are reduced, and their aspect ratio increases, penetration of DI water and liquid solvents into these features becomes more difficult due to surface tension issues. Also, if liquids do penetrate into such small features, then it becomes increasingly difficult to subsequently remove. Consequently, wet processes have limitations in cleaning residue from the bottom of high-aspect ratio features with small lateral dimensions. Additionally, these wet cleaning methods can over etch exposed layers. This can cause device reliability problems or lead to nonfunctional circuits. And although the oxidative chemistry component of some traditional wet clean methods (e.g. dilute HF (50:1-1000:1 HF:H2O) or xe2x80x9chot Piranhaxe2x80x9d (90% H2SO4/10% H2O2)) can be effective at cleaving the bonding structures of contaminant residues, often the formulations and or cleaning conditions do not provide efficient physical removal of the contaminants. These methods also have the disadvantage of requiring handling and exposure to corrosive and flammable media, thus requiring extensive abatement and environmental controls.
Also there are a number of emerging methods for cleaning wafers. Amongst these new methods, high-pressure processes that employ local densification of a process fluid on the substrate hold promise. Densified fluids are good solvents for contaminants and residues resulting from semiconductor fabrication. Some of these processes, especially those conducted at supercritical pressures, employ additives to increase the solvating power of the process fluid itself. Other additives are used to remove specific contaminants such as polymers, organics, metals, and the like. Although supercritical fluids hold promise for wafer processing, more can be done to exploit their valuable physical properties.
What is therefore needed are improved methods for removing contaminants from wafers, for example photoresist and post-etch residues, preferably methods that not only effectively clean wafers, but also condition them for subsequent processing.
Methods for cleaning semiconductor wafers are presented. Contaminants, particularly photoresist and post-etch residue, are removed from semiconductor wafers. A wafer or wafers is first treated with a peroxide-containing media, for example, to oxidatively cleave bond structures of contaminants on the wafer work surface. Excitation energy is used to activate the peroxide-containing medium toward the formation of radical species. After treatment with the peroxide-containing medium, a supercritical fluid treatment is used to remove any remaining contaminants as well as condition the wafer for subsequent processing.
One aspect of the invention is a method of cleaning a wafer work surface. Such methods may be characterized by the following operations: applying a peroxide-containing medium to the wafer work surface; applying an excitation energy to the peroxide-containing medium while the peroxide-containing medium is in contact with the wafer work surface, the excitation energy sufficient to generate corresponding hydroxy and peroxy radicals from interaction with the peroxide-containing medium; and then applying a supercritical fluid to the wafer work surface.
Preferably the peroxide-containing medium includes at least one of an aqueous solution, an organic solution, and a solution containing both water and an organic solvent. Preferably the organic solvent includes at least one of an ether, an alcohol, an alkyl halide, a ketone, a nitrile, an aliphatic solvent, an aromatic solvent, an amide, an ester, an acid, an amine, and a fluorinated alkane. Most preferred organic solvents include but are not limited to acetonitrile, ethanol, methanol, isopropanol, tetrahydrofuran, methylene chloride, chloroform, 1,2-dichloroethane, diethyl ether, hexane, toluene, benzene, xylene, tertiary butyl methyl ether, 1,4-dioxane, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethylene glycol, propylene glycol, ethyl lactate, acetic acid, trifluoroacetic acid, dimethylacetimide, N-methylpyrrolidinone, dimethyl formamide, dimethyl ethanolamine, and, hexafluoroethane.
Also preferably the peroxide-containing medium includes between about 10% and 70% by weight of a peroxide source, more preferably between about 30% and 50% by weight of a peroxide source. In preferred embodiments, the peroxide source includes at least one of an inorganic peroxide and an organic peroxide. Inorganic peroxides preferably include at least one of hydrogen peroxide and peroxide adducts such as urea hydroperoxides, ammonium persulfate, and sodium percarbonate. Organic peroxides of the invention include at least one of a monoalkyl peroxide, a dialkyl peroxide, a monoacyl peroxide, and a diacyl peroxide. Specific examples of such organic peroxides include at least one of tertiary butyl hydrogen peroxide, meta-chloroperbenzoic acid, benzoyl peroxide, di-tertiary butyl peroxide, dicumyl peroxide, acetyl peroxide, benzyl peroxide, and butanone peroxide.
The peroxide-containing medium preferably further includes a radical initiator and/or ozone. Preferably the radical initiator includes at least one of 2,2xe2x80x2-azo-bis-isobutyrlnitrile, dicumyl peroxide, benzoyl peroxide, and the like
The pH of the peroxide-containing medium is important If basic, preferably the pH of the peroxide-containing medium is between about 9 and 12. If acidic, preferably the pH of the peroxide-containing medium is between about 1 and 6.
Preferably applying an excitation energy to the peroxide-containing medium while the peroxide-containing medium is in contact with the wafer work surface includes application of at least one of a heat source, an irradiation source, and a mechanical agitation source to at least one of the wafer and the peroxide-containing medium.
Irradiation sources of the invention preferably include at least one of a UV lamp, a mercury arc lamp, an eximer laser, a xenon flash lamp, and a high intensity discharge lamp. In one preferred embodiment, applying the excitation energy includes exposing the peroxide-containing medium to ultraviolet irradiation with a wavelength of between about 10 nm and 500 nm, more preferably between about 150 nm and 200 nm.
Heat sources of the invention preferably include at least one of a wafer stage heater, an infrared heater lamp source, a process vessel with heated interior surfaces, and a recirculating heater or heat exchange coils immersed in the processing fluid. In such embodiments, preferably applying the excitation energy includes heating the peroxide-containing medium to between about 40xc2x0 C. and 150xc2x0 C.
Preferably the mechanical agitation sources of the invention include at least one of a transducer element (capable of generating mechanical vibration) and a probe member to transmit (either directly or indirectly) said mechanical vibration to the wafer. Mechanical agitation sources may also include a rotation mechanism, an orbit mechanism, and the like.
For the transducer elements of the invention, preferably the source of such vibration may operate in either the ultrasonic or megasonic spectrum. The ultrasonic spectrum would be characterized by frequencies in the range between 10 kHz and 40 kHz, while the megasonic spectrum would consist of frequencies in the range between 100 kHz and 1 MHz.
Preferably the mechanical agitation includes motion sufficient to remove spent reagent from the wafer surface. Such motion may also serve to position the wafer such that the entire work surface area is under the influence of the excitation energy sources. An example of such motion is rotation or orbiting about an axis perpendicular to the work surface of the wafer. Orbiting motion contains both a rotational and a translational element. Preferred speeds of rotation (whether orbiting or not) during exposure to either the peroxide-containing medium or the supercritical fluid preferably are between about 2 rpm and 200 rpm, more preferably between about 10 rpm and 50 rpm. During any subsequent drying processes, for example to remove liquid reagent from the wafer surface after all resist has been stripped, preferably the rotation speeds are between about 100 rpm and 5000 rpm, more preferably between about 1000 rpm and 3000 rpm.
One skilled in the art would understand that any individual excitation energy application may include any or all of the above mentioned energy sources. Particularly preferred embodiments are described in more detail below.
Preferably applying the peroxide-containing medium to the wafer work surface and applying the excitation energy to the peroxide-containing medium while the peroxide-containing medium is in contact with the wafer work surface are conducted over a period of not more than about 60 minutes, more preferably not more than about 10 minutes, most preferably not more than about 2 minutes.
Methods of the invention can further include rinsing the wafer work surface with a solvent before applying the supercritical fluid to the wafer work surface. Deionized water is a particularly preferred solvent for this purpose, although other solvents, for example organic solvents as listed above or mixtures thereof, can be used for rinsing the wafer.
Preferably applying the supercritical fluid to the wafer work surface is performed at a pressure of between about 1200 and 5000 psi, and at a temperature of between about 20xc2x0 C. and 150xc2x0 C. Preferred supercritical fluids of the invention include at least one of carbon dioxide, ammonia, water, ethane, propane, butane, dimethyl ether, hexafluoroethane, dimethyl ether, SF6, ethylene, N2O, Xe, and mixtures thereof.
In particularly preferred embodiments, the supercritical fluid also contains between about 0 and 15% by weight of an additive. Additives of the invention include, but are not limited to at least one of acetonitrile, ethanol, methanol, isopropanol, tetrahydrofuran, methylene chloride, chloroform, 1,2-dichloroethane, diethyl ether, hexane, toluene, benzene, xylene, tertiary butyl methyl ether, 1,4-dioxane, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethylene glycol, propylene glycol, ethyl lactate, acetic acid, trifluoroacetic acid, dimethylacetimide, N-methylpyrrolidinone, dimethyl formamide, dimethyl ethanolamine.
Also preferably, applying the supercritical fluid to the wafer work surface includes passing the supercritical fluid through a process vessel containing the wafer such that a flow field is established over the wafer work surface. Preferably the flow field impinges on the wafer with a flux of between about 100 g/min and 10 kg/min. In a particularly preferred embodiment, the flow field is established over each work surface of a plurality of wafers. Preferably the plurality of wafers is at least about twenty-five wafers (i.e. an industry recognized standard for a cassette of wafers). Preferably at least one of the supercritical fluid and the wafer are agitated while the supercritical fluid is in contact with the wafer work surface. Agitation preferably includes at least one of pulsing the pressure of the supercritical fluid, sonicating, vibrating, stirring, high flow, fluid recirculation, and combinations thereof. In preferred embodiments, applying the supercritical fluid to the wafer work surface is conducted over a period of not more than about 60 minutes, more preferably not more than about 20 minutes, most preferably not more than about 5 minutes.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated drawings.